Hi,
Blockchain Wallet API gives back Unknown Error recently if I try to send with hex encoded private key:
https://blockchain.info/merchant/4E533581DF08E14AC890C861F79F35F44CE7001289C6DAB1DC98FCF93E6329D5/payment?to=1EgUYuEwPoZXgniNA66Hq8Y4Q6uCEyvXH&amount=1
Response:
{"error":"Unknown Error"}